The selective removal of oil from wastewaters while minimizing concentrate production using a membrane cascade

This paper describes the results of a study aimed at removing oil and grease from bilge water accumulating in the lower recesses of ships. Bilge water is a difficult wastewater to treat as it contains seawater, particulates, used oils and detergents. The objective of this work was to design a membrane system that minimized the production of brine retentate waters that must be stored onboard a ship. A pilot scale membrane cascade system was designed and evaluated in this work using tubular MF and UF membranes in a first stage, where high levels of particulate matter are present, and Sepa® flow cells representing spiral wound modules in the second stage. Large channel MF and UF ceramic membranes were used in the first loop while a variety of flat sheet UF (1–100 kD MWCO) and NF membranes were studied in the second stage (loop 2). Permeates obtained from the first and second stages were analyzed using an extraction procedure followed by gas chromatography. It was found that lower MWCO membranes (1–5 kD) had excellent permeate fluxes ranging from 25 to 62 L/m2/h and rejections rates of hexane extractable organics were above 90%. The hexane extractable oil and grease concentrations in the permeates from these membranes were found to be 0.1 and 0.0 ppm for the corresponding 1, and 2 kD MWCO membranes. The size of oil particles in the second loop was measured by dispersive light scattering (DLS). Nano sized particles were observed and the size of these particles was found to increase over time as a result of mixing and circulating in the second loop.